I. Field of the Invention
The present invention relates to techniques for semiconductor processing, and more particularly to semiconductor processing which may be performed at low temperatures.
II. Description of the Related Art
In the field of semiconductor processing, there have been several attempts to use lasers to convert thin amorphous silicon films into polycrystalline films. An overview of conventional excimer laser annealing technology is presented by James Im et al. in xe2x80x9cCrystalline Si Films for Integrated Active-Matrix Liquid-Crystal Displays,xe2x80x9d 11 MRS Bulletin 39 (19%). In systems used for carrying out excimer laser annealing, an excimer laser beam is shaped into a long beam which is typically up to 30 cm long and 500 micrometers or greater in width. The shaped beam is scanned over a sample of amorphous silicon to facilitate melting thereof and the formation of polycrystalline silicon upon resolidification of the sample.
The use of conventional excimer laser annealing technology to generate polycrystalline or single crystal silicon is problematic for several reasons. First, the silicon generated in the process is typically small grained, of a random microstructure, and/or has non-uniform grain sizes, which result in poor and non-uniform devices that lead to low manufacturing yield. Second, the processing techniques needed to obtain acceptable performance levels require that the manufacturing throughput for producing polycrystalline silicon be kept low. Also, these processes generally require a controlled atmosphere and preheating of the amorphous silicon sample, which lead to a further reduction in throughput rates. Finally, the fabricated films generally exhibit an unacceptable degree of surface roughness that can be problematic for performance of microelectronic devices.
There exists a need in the field to generate higher quality polycrystalline silicon and single crystal silicon at greater throughput rates. As well, there exists a need for manufacturing techniques that reduce the surface roughness of such polycrystalline and single crystal silicon thin films to be used in the fabrication of higher quality devices, such as flat panel displays.
An object of the present invention is to provide techniques for planarizing the surfaces of polycrystalline and single crystal thin film semiconductors.
A further object of the present invention is to provide surface planarization techniques that may be applied as a post processing step to polycrystalline and single crystal thin film semiconductors that are produced during a sequential lateral solidification process.
Yet a further object of the present invention is to provide surface planarization techniques that may be applied as a processing step during the production of polycrystalline and single crystal thin film semiconductors in a sequential lateral solidification process.
Yet another object of the present invention is to provide techniques for the fabrication of high quality semiconductors devices useful for fabricating displays and other products.
In order to achieve these objectives as well as others that will become apparent with reference to the following specification, the present invention provides systems and methods for reducing surface roughness of a polycrystalline or single crystal thin film that had previously been produced by the sequential lateral solidification process. In one arrangement, the system includes an excimer laser for generating a plurality of excimer laser pulses of a predetermined fluence, an energy density modulator for controllably modulating the fluence of the excimer laser pulses such that the fluence is below that which is required to completely melt the thin film, a beam homogenizer for homogenizing modulated laser pulses in a predetermined plane, a sample stage for receiving homogenized laser pulses to effect partial melting of portions of the polycrystalline or single crystal thin film corresponding to the laser pulses, translating means for controllably translating a relative position of the sample stage with respect to the laser pulses, and a computer for coordinating the excimer pulse generation and fluence modulation with the relative positions of the sample stage to thereby process the polycrystalline or single crystal thin film by sequential translation of the sample stage relative to the laser pulses. The excimer laser is preferably an ultraviolet excimer laser for generating ultraviolet excimer laser pulses.
In one arrangement, the beam homogenizer is operable to shape laser pulses with a tophat profile in both the x and y directions. The energy density modulator is operable to attenuate fluence of the excimer laser pulses to approximately 25% to 75% of the full melt threshold of the polycrystalline or single crystal thin film.
The translating stage advantageously includes an X direction translation portion and a Y direction translation portion, each being coupled to the computer and to each other and permitting movement in two orthogonal directions that are perpendicular to a path formed by the laser pulses, and being controllable by the computer for controllably translating the sample in both of said translatable directions under control of said computer. Also, the beam homogenizer is operable to shape said laser pulses with a tophat profile in both the x and y directions, and the translating means is operable to translate the polycrystalline or single crystal thin film in two directions orthogonal to a direction of said laser pulses such that sequential homogenized laser pulses are incident on slightly overlapping regions of the polycrystalline or single crystal thin film in the two directions.
In an alternative arrangement, the present invention provides for systems and methods for processing an amorphous silicon thin film sample into a single or polycrystalline silicon thin film having a reduced surface roughness. In one arrangement, the method includes forming a rigid cap layer on an amorphous silicon thin film sample having sufficient thickness to withstand contractions and expansions during melting and resolidification of the silicon thin film during the sequential lateral solidification process. The method also includes generating a sequence of excimer laser pulses; controllably modulating each excimer laser pulse in the sequence to a predetermined fluence; homogenizing each modulated laser pulse in the sequence in a predetermined plane; masking portions of each homogenized fluence controlled laser pulse in the sequence to generate a sequence of fluence controlled pulses of patterned beamlets, irradiating the amorphous silicon thin film sample with the sequence of fluence controlled patterned beamlets to effect melting of portions thereof; controllably sequentially translating the sample relative to each of said fluence controlled pulse of patterned beamlets to thereby process the amorphous silicon thin film sample into a single or polycrystalline silicon thin film having a reduced surface roughness; and removing said cap layer from the processed single or polycrystalline silicon thin film.
The accompanying drawings, which are incorporated and constitute part of this disclosure, illustrate a preferred embodiment of the invention and serve to explain the principles of the invention.